Drive circuits for Power MOSFETs and IGBTs

by B. Maurice, L. Wuidart
Unlike the bipolar transistor, which is current driven,
Power MOSFETs, with their insulated gates, are
voltage driven. A basic knowledge of the principles
of driving the gates of these devices will allow the
designer to speed up or slow down the switching
speeds according to the requirements of the
It is often helpful to consider the gate as a simple
capacitor when discussing drive circuits.
2.1 Gate vs Base
Power MOSFETs and IGBTs are simply voltage
driven switches, because their insulated gate
behaves like a capacitor. Conversely, switches such
as triacs, thyristors and bipolar transistors are
“current” controlled, in the same way as a PN diode.
A remarkable effect can be seen in both the turn-on
and turn-off switching waveforms; the gate voltage
exhibits a “step”, remaining at a constant level while
the drain voltage rises or falls during switching. The
voltage at which the gate voltage remains during
switching is known as the Miller voltage, Vgm. In
most applications, this voltage is around 4 to 6V,
depending on the level of current being switched.
This feature can be used to control the switching
waveforms from the gate drive.
2.3 MOSFET and IGBT turn-on / turn-off.
When turned on under the same conditions, IGBTs
and MOSFETs behave in exactly the same way,
and have very similar current rise and voltage fall
times - see figure 3.
2.2 Driving a gate
However, at turn-off, the waveforms of the switched
current are different, as shown in figure 4. At the end
of the switching event, the IGBT has a “tail current”
which does not exist for the MOSFET.
As shown in figure 2, driving a gate consists of
applying different voltages: 15V to turn on the device
through S1, and 0V to turn off the device through S2.
This tail is caused by minority carriers trapped in the
“base” of the bipolar output section of the IGBT
causing the device to remain turned on. Unlike a
Figure 1. Nature of power semiconductor inputs
Figure 2. Driving MOSFET / IGBT gates
Figure 3. MOSFET / IGBT turn-on
Figure 4. MOSFET / IGBT turn-of
bipolar transistor, it is not possible to extract these
carriers to speed up switching, as there is no external
connection to the base section, and so the device
remains turned on until the carriers recombine
naturally. Hence the gate drive circuit has no effect
on the tail current level and profile. The tail current
does however increase significantly with
2.4 IGBT turn-off losses
The turn-off of an IGBT can be separated into two
distinct periods, as shown in figure 5. In the first
period, its behaviour is similar to that of a MOSFET.
The increase in drain voltage (dV/dt) is followed by a
very fast fall of the switched current. Losses in this
“dV/dt” period depend mainly on the speed of the
voltage increase, which can be controlled by a gate
drive resistor.
The second “tail current” period is specific to the
IGBT. As this period occurs while there is already a
large voltage across the device, it causes losses at
each turn-off.
The total turn-off losses are shown in figure 5 by the
shaded area.
losses is linked to the switching frequency. Turn-off
losses become critical when operating at high
frequencies. In this case, the dV/dt can be increased
(and hence losses reduced) by decreasing the size
of the gate drive resistor Rg, which will allow the gate
to charge more quickly. The turn-off losses are
proportional to the size of the gate resistor - for
example decreasing the gate resistor from 100 to 10
Figure 5. IGBT turn-off losses
dV/dt losses
Tail losses
3.1 Speeding up turn-off
The power involved in these two types of switching
will reduce the dV/dt losses by a factor of 10 - see
figure 6.
However, it should be remembered that IGBT tail
current losses are completely independent of the
value of the gate resistor.
It can be noted that in figure 6 the dV/dt and tail
current losses are around the same with a gate
resistance of 47Ω.
Even though the tail current is constant, the losses
in a system are often predominantly due to dV/dt,
because the value of the gate resistance is often too
high. In the example of figure 7, the total losses per
cycle are reduced from 13mJ to 4mJ by decreasing
the gate resistance from 100Ω to 10Ω.
3.2 Reducing dV/dt at turn-off
Conversely, in low frequency applications, fast
switching waveforms can cause problems in the
form of EMI. A gate driven switch can be used to
reduce the amount of EMI, by slowing down the
switching speed. This is particularly useful in
applications where the mains phase angle is
The dV/dt can be expressed as:
dV =
(Rg . Ciss)
where Vgm (the Miller gate voltage) is around 6V, Crss
is the equivalent gate-drain capacitance and Rg is
Figure 6. Speeding up turn-off
VG =
VD =
ID =
T =
25A - 1000V
Figure 7. Variation of turn-off losses with gate
the value of the gate resistor at turn-off. One method
of slowing down the switching is thus to slow the
rate at which the gate capacitor is charged - see
figure 8. This can be achieved using a large gate
resistor to make the gate charge more slowly and
hence increase the dV/dt time. Throughout the dV/dt
period, the voltage across the gate resistor is equal
to the Miller voltage (Vgm), and for a short time the
power switch operates in linear mode. In this
example, with a STGP10N50 IGBT (Crss≈ 40pF) the
dV/dt will be around 7.5V/µs.
Alternatively, a capacitor can be connected between
the gate and collector / source of the device, which
increases the capacitance which must be discharged
through the gate resistance at turn-off.
Figure 8. Slowing down turn-off using a gate resistor
3.3 Reducing dI/dt at turn-off and turn-on
A technique which slows both turn-on and turn-off
uses a small inductor lE placed in the emitter/source
lead of the device, as shown in figure 9. The voltage
e developed across the inductor during switching,
given by:
e = lE . dI
drive voltage and the Miller gate voltage (Vgm, around
6V). The value of dI/dt can thus be calculated as:
(Vg - Vgm)
For example, in the 4kW example shown in figure 9,
at turn-off (Vg = 0V) dI/dt = -6V / 3µH = -2A/µs. To
give an idea, in the circuit used in this example the
switching losses are only around 0.8W.
must be equal to the difference between the gate
Figure 9. Slowing down the switching of the current using a feedback inductor
dI/dt ON = 5A/µs
dI/dt OFF = 1.5A/µs
4.1 Gate as memory
The capacitive nature of the gate input can be
exploited in many different ways, for example as a
In the circuit of figure 10 a single voltage pulse
applied to the gate through diode D1 is sufficient to
charge the input capacitance Cin and turn on the
switch T1. When the pulse has finished, D1 prevents
the gate discharging, and so the device remains on:
the gate is behaving as a memory of the on-state of
the switch. To “erase” the gate memory and turn off
the switch, a pulse is applied to the diode D2 which
turns on T2, which in turn discharges the gate of T1
and turns the device off. As T2 remains on, T1
cannot be accidentally turned on due to dV/dt effects,
and so the gate of T2 is now behaving as a memory
of the off state of T1.
As the pulse duration times required to turn the
devices on and off are very small, this principle can
be adapted to suit a wide variety of switching
frequencies: from almost continuous operation up to
In low frequency applications, refresh pulses can be
used to prevent the gate capacitor discharging due
to leakage currents.
Figure 10. Using the gate as a state memory
The major benefit of this technique lies in the very
small size of the pulse transformer required for a
wide range of switching frequencies.
For further information on this subject, see
reference 1.
4.2 Using the gate in resonant circuits
The gate capacitor can also be used as part of a
resonant LC network - see figure 11. With the same
peak current value, the capacitor is charged around
twice as fast with an inductor compared to a resistor.
If the resistor is replaced with an inductor, losses in
the gate drive resulting from the charge and discharge
current of the gate capacitor become negligible. This
solution is particularly efficient in very high frequency
applications where gate drive losses become more
An additional benefit is that a resonant circuit has an
inherent voltage step-up ability, which means that
the 15V required to drive the gate can be generated
from a much lower voltage.
Figure 12 shows an example of the gate capacitance
being used as part of a resonant circuit.
This type of solution is mainly of use in drive circuits
of high power MOSFETs which interface directly
with standard 5V CMOS microcontrollers.
Figure 11. Using the gate capacitance in a resonant circuit
Figure 12. Resonant gate drive circuit
4.3 The gate as an EMI reducer
As mentioned above, the switching waveforms of
Power MOSFETs and IGBTs can easily be slowed
by adjusting the value of the gate resistor. This
feature can be used as an EMI reducer in applications
where the mains phase angle is switched (figure
13), for example light dimmer circuits.
Conventional dimming circuits are controlled by
TRIACs. Turning a TRIAC on or off generates voltage
spikes and uncontrolled dV/dt. In most cases a
TRIAC requires a series inductor for EMI filtering.
When the power is controlled by an IGBT, the
Figure 13. EMI reduction
Figure 14. Soft light dimmer circuit
switching behaviour can be softened at both turn-on
and turn-off so that the inductor is no longer required.
The switching losses incurred by slowing down the
turn-off of the IGBT are not critical at mains frequency.
The soft light dimmer shown in figure 14 and
discussed in reference 2 is based on the use of an
IGBT as a switch whose turn-off may be controlled.
Such a circuit allows the current switching slopes to
be controlled, removing the need for an EMI filter,
reducing costs and eliminating the associated
acoustic noise. Short circuit protection can also be
built in, which means that a fuse is no longer required.
4.4 Automatic floating gate drive
Another useful feature resulting from the small size
of the gate capacitor is the low drive energy required
to switch high current levels. This characteristic has
been used for automatic floating gate drives in
asymmetrical half bridges - see reference 3.
Because the drain/emitter voltage of the high side
switch in an asymmetrical half bridge floats, most
applications require an additional pulse transformer
to drive it. In most cases this pulse transformer
provides the isolation required to interface the high
side switch with the ground-connected PWM control
However, in the circuit shown in figure 15 an auxiliary
winding of the power transformer is used to drive the
high side switch as a synchronized slave of the
grounded low side switch; when the low side switch
turns on or off, the high side switch is automatically
turned off or on.
This circuit removes the need for a pulse transformer,
and works with very few external components.
VCE voltage drop will typically be around 2 - 3V, but
this increases with increasing collector current. The
Zener diode Z1 is selected to set the VCE level at
which the protection will operate.
Consequently, in the normal mode of operation, 15V
is applied to the input to turn the transistor fully on,
which also causes the diode D to be forward biassed
through resistor R1. The voltage at point P is thus
equal to the VCE voltage drop across the IGBT, plus
the voltage drop across D. The rating of Z1 is chosen
such that in these conditions it remains blocked.
However, if an overcurrent causes the VCE of the
IGBT to increase, when the voltage at point P reaches
the rating of the Zener Z1, Z1 begins to conduct,
turning on T2, and clamping the voltage at point P,
causing D to become reverse biassed. Turning on
T2 causes Zener Z2 to clamp the IGBT gate voltage
at 6V, limiting the collector current to a lower level.
Figure 16. Current limitation using multiple drive
4.5 Using multiple drive voltages
The drive circuit shown in figure 16 takes advantage
of the voltage driven nature of the gate. In normal
operation, 15V is applied to drive the gate fully on,
but if an overcurrent is detected, the gate voltage is
clamped at 6V (the Zener diode voltage of Z2),
limiting the collector current.
Overcurrent is detected by monitoring the collectoremitter voltage of the IGBT - in normal operation the
R1 P
Figure 15. Automatic floating gate drive
Although insulated gate devices are widely used
and well understood, it remains interesting to
reconsider the gate operating as a simple capacitor.
A useful feature of insulated gate switches is their
ability to soften switching waveforms easily. IGBTs
used in this way, as EMI reduction / turn-off
controllable switches, are a very attractive alternative
to TRIACs in lamp dimming circuits.
1] A New Isolated Gate and Base Drive for
Power MOSFETs and IGBTs
J.M. Bourgeois
SGS-THOMSON Microelectronics
application note AN461
Additionally, the ability of insulated gate switches to
be driven with a small amount of energy has lowered
the power level at which half bridge topologies can
effectively be used. This trend of the last decade is
highlighted demonstrated by the advent of integrated
high side driver circuits. For this reason, equipment
designers will no longer hesitate to drive high side
floating Power MOSFETs and IGBTs, even in the
100W power range.
2] Soft Light Dimmer
J.M. Charreton
SGS-THOMSON Microelectronics
application note AN518
3] Ultra Fast Ni-Cd Battery Charger
L. Wuidart, J.M. Ravon
SGS-THOMSON Microelectronics
application note AN486
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